The enteric nervous system (ENS) is a complex network of neurons and glia within the bowel wall that is essential for normal intestinal function. When ENS development is abnormal, serious problems with intestinal motility occur, including Hirschsprung's disease (distal intestinal aganglionosis) and intestinal pseudo-obstruction. Less severe, but more common problems with ENS structure or function may also contribute to many cases of irritable bowel syndrome. Defects in Ret tyrosine kinase signaling are the most common cause of Hirschsprung's disease and may result in a wide variety of abnormalities in ENS structure and function. Ret signaling is essential for ENS precursor survival, proliferation, migration, and axon extension. In addition, Ret activation influences neuronal activity both in vitro and in vivo. Studies in model cell culture systems have identified many intracellular signaling pathways activated by Ret. Despite these advances, many questions remain. The importance of specific Ret signaling pathways for ENS development is not yet known and factors that influence the penetrance of Hirschsprung's disease in people with Ret mutations are poorly understood. Specific Aim I will take advantage of a large number of mouse mutations to determine the importance of specific Ret signaling pathways for ENS development in vivo. Specific Aim II will evaluate the importance of these same signaling pathways using ENS precursors grown in primary culture. These studies will define how specific Ret activated signaling systems influence ENS precursor survival, proliferation, migration and axon extension. Specific Aim III will determine why vitamin A signaling is important for ENS development and identify interactions between retinoid and Ret signaling. All of the proposed experiments are designed to provide more detailed information about mechanisms of ENS development that may lead to novel treatment strategies for human motility disorders.